Helix structure for traveling-wave tubes



Sept. 2, 1958 G. R. BREWER HELIX STRUCTURE FOR TRAvELING-WAVE TUBESFiled Dec. 1, 1955 a a', 0 I l -K www.. ,imei/VIK hired Patented SephZ,1958 lillElLliX STRUCTURE EUR TRAVELlNG-WVE TUBES Appiication Decemberi, 1955, Serial No. 550,304

3 Ciaims. (Cl. S15-3.5)

This invention relates generally to traveling-wave tubes i andparticularly relates to an improved helix structure having high powerdissipation capability.

A. traveling-wave tube conventionally consists of a slow-wave structuresuch as a helix along which electromagnetic waves are caused topropagate in a manner such that the velocity of propagation in the axialdirection is substantially less than the velocity of light. Then alongthe interior of the helix an electron stream is projected at a velocityapproximately that or the axial velocity of the traveiing waves alongthe helix. An interchange of energy is thus possible between theelectron stream and the traveling-wave. In the case of a forwardwaveamplifier the electron stream velocity is caused to be slightly greaterthan the velocity of axial propagation of the traveling-waves along thehelix and electromagnetic pushing of the traveling-waves results in anamplication thereof. ln a backward-wave amplier the electron stream iscaused to amplify a traveling wave propagating in a direction oppositeto the direction of travel of the electron stream; and the backward wavechosen to be so amplified corresponds to that Fourier component of aforward traveling wave which would be amplified along with thefundamental forward traveling wave.

In any such traveling-wave tube having maximum efficiency of operationthe electron stream is caused to traverse the length of the helix andpass as closely as possible to the helix itself in order to achievemaximum coupling between the traveling-waves andthe electron stream. Ifthe electron stream is caused to pass too closely to the helix, anexcessive proportion of the electrons in the stream impinge upon thehelical conductor and excessively heat it which in turn decreases theefciency of the tube and endangers it mechanically due to thermalexpansion. Grdinarily an optimum is sought between very close couplingof the electron stream to the helix and keeping the electro-n streamseparated from the helix so as to create no serious heat dissipationproblems.

in order to allow closer coupling and therefore more 5 heating effect,the outside of the helix is often cooled by the passage of a suitablecoolant tiowing as nearly as possible to the outside of the helix.Ordinarily, however, the helical conductor is supported within avacuumtight glass envelope, the heat conducting limitations of whichseverely reduce the efficiency of heat exchange between helix andcoolant.

it is therefore the principal object of this invention to provide, in ahigh power traveling-wave tube, a helix which may be cooled by the flowof a coolant in a manner such that the heat to be dissipated need notdow through an envelope.

Briefly, in accordance with this invention this and other objects areachieved by providing a combined helical cou-- ductor and envelope forvacuum purposes in which the helix is adjacent to the beam (heat source)on the inside and exposed directly to the coolant (heat sink) on theoutside. A helix is provided which is filled and sealed between itsturns by a dielectric material such as glass or a ceramic to therebyprovide a vacuum tight helix serving as an envelope.

The novel features which are believed to be characteristic of thisinvention, both as to its organization and method of operation, togetherwith further objects and advantages thereof, will be better understoodfrom the following escription considered in connection withktheaccompanying drawing in which several embodiments of the invention areillustrated by way of example. It is to be expressly understood,however, that the drawing is for the purpose of illustration anddescription only, and is not intended as a definition of the limits ofthe invention.

In the drawing:

Fig. l is a schematic View, partly in section, of a traveling-wave tubeembodying, in accordance with the present invention, a combined helixand a Vacuum envelope.y

Fig. 2 is a sectional View of a portion of the slow-wave structure andenvelope of Fig. l.

Figs. 3 and 4 are sectional views of alternative embodiments of aportion of an envelope at one stage of its manufacture.

Fig. 5 is a sectional view of a portion of a combined helix and Vacuumenvelope in accordance with the present invention.

Referring now to the drawing and particularly to Fig. l in which theinvention is embodied in a forward-wave amplier as an example.Traveling-wave tube 10 is shown surrounded along most of its length by asolenoid l2. At the left hand end of tube iti an enlarged glass portioni4 is provided for housing an electron gun i6 which includes a cathode18, a focusing electrode 20, and an accelerating anode 22. A ycollectorelectrode 24% is disposed at the right hand end of tube l0 forintercepting the electron beam emitted by electron gun 16. Disposedbetween electron gun i6 and collector electrode 24 and surrounded bysolenoid 12 is a combined slow-wave structure and envelope 26. A helix28'which may be made of a metallic wire or ribbon is axially alignedcontiguous to the path of the electron stream emitted by electron gunle. interposed between the turns of helix 28 is a dielectric material 30such as glass or ceramic which is bonded to the helix continuously alongits length and which is sealed to glass housing i4 at its left extremityand to collector electrode 24 at its right extremity to provide aVacuum-tight envelope for the electron beam. Coupled in a conventionalmanner .to the emitter end of helix 28 is an input transmission line 32.In like man# ner an output transmission line 34 is coupled to the colnlector end of helix 28.

Between so-lenoid i2 and helix .2S a cylindrical passage is provided forthe liow of coolant which is pumped or blown by a suitable deviceindicated schematically by a pump 36. Appropriate D. C. voltage sourcesnecessary for the operation of tube lt? are shown schematically by 3S atthe electron gun end of tube l@ and at te at the collector end and at 42for the solenoid.

It is seen that the coolant forced by pump 36 passes directly over helix28. The electrical properties of the coolant inherently affect thepropagation characteristicv of helix 28 in that the dielectricproperties of the coolant may change the velocity of propagation and theimpedance of the helix while the loss characteristic of the coolantintroduces loss to the wave propagating on the helix. However, it hasbeen found that practically any gas is satisfactory and is in fact animprovement over the conventional glass envelope as regards its lossfactor. Almost any liquid having mobility and heat transfer propertiestogether with low dielectric coolant and loss is also satisfactory. Forexample, in the order of incoolant as a heat sink is caused to flowcontiguously to the outside surface of the helix in counter current heatexchange'.

' In Fig. 3 there is shown in section a portion of a ceramic or glasscylinder 43 which has been provided with a spiral groove 44 along itsinner surface` Fig. 4 shows a similar cylinder 47 with a spiral groove4S provided in its outer surface. The combined helix and glass envelopeof this invention may be provided by threading either of these grooves44 or 4S with helix 28. The helix 23 may then be soldered in place orotherwise bonded to the glass or ceramic 43 or 47. In the example shownin Fig. 3 the excess glass outside the helix is then ground away to formthe finished vacuum-sealed helix 26 as shown in Fig. 5. Alternatively ifthe example of Fig. 4 is used the interior excess glass is ground awayso that the elec tron beam. may pass contiguous to helix 28 and provideagain the structure as shown in Fig. 5.

An important advantage inherent in the slow-wave structure of thepresent invention is that the critical spacing or interstices betweenthe turns of the helix is securely maintained by the interposed rigiddielectric.

A further method in which the structure of this invention may beprovided would be to wind helix 28 upon a mandrel and then wind betweenits turns a helix or" softened malleable glass having the samethickness-as the wire or ribbon of helix 28 to thus eliminate the stepof grinding away excess glass.

Still another method is to wind helix 23 upon amandrel, shrink thereovera glass cylinder, and-then grind away excess glass outside the helix sothat coolant may be passed in direct contact with the helix over itsoutside surface.

In the operation of the tube of the present invention electron gun 16emits a beam which is caused toV pass as nearly as possible to the innersurface of helix 28 as the stream traverses the tube toward collector24. The electrons may pass as closely as desired to the helix becausethe coolant forced by pump 36 passes very efficiently in direct contactwith the outside surface of helix 28 to therebyv increase by a factor ofmore than 10 the heat transfer from helix to coolant.

There has thus been disclosed a combined helical slow-wave structure andvacuum envelope in which the electron stream of a high-powertraveling-wave tube may pass as closely as desired to the interiorsurfaces of the slow-wave structure while allowing a coolant to passdirectly over the outside surface of the helix to thus greatly increasethe eiciency and power dissipating property of a traveling-wave tube.

What is claimed is:

1. A traveling-wave tube comprising: means for projecting an electronbeam along a predetermined path; a collector electrode for interceptingsaid beam; and a combined slow-wave structure and vacuum envelopedisposed about said path, said slow-wave structure including at leastone helical conductor directly exposed on its inside surface to saidelectron stream and directly exposed on its outside surface to acoolant, and dielectric material v in the interstices between successivehelical conductors and hermetically bonded to said conductors.

2. A traveling-wave tube comprising: means for projccting an electronbeam along a predetermined path and a combined helical slow-wavestructure and vacuum en-l velope consisting of a composite cylinderdisposed about said path in axial alignment therewith for propagatingelectromagnetic waves therealong in energy exchange relation with saidelectron beam and comprising alternate axial segments of a helicaldielectric ribbon and a helical metallic ribbon, said dielectric ribbonand said metallic ribbon being bonded together with the dielectricribbon being interposed between successive turns of the metallic ribbonand joined hermetically thereto to provide a cylinder whose wall isvacuum tight and the outer surface of which consists alternately of saiddielectric and metallic ribbons.

3. A traveling-wave tube comprising: means for projecting an electronbeam along a predetermined path; a collector electrode for interceptingsaid beam; and a combined helical slow-wave structure and vacuumenvelope disposed about said path including a helical di electric ribbonand a helical metallic ribbon, said dielectric ribbon and said metallicribbon having substantially equal radial thickness when assembled in ahelix hermetically bonded together to provide a composite cylinder whosewall is vacuum tight, with the outer periphery of said helical metallicribbon forming a part of the outside of the wall.

References Cited in the file of this patent y UNITED STATES PATENTS1,797,990

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